Force control of robotic mechanisms continues to be a challenging area. Previous implementation have seldom produced satisfactory results, and researchers in the past have experienced significant instability problems associated with their force controllers. In this study, a new stability factor in force control will be pointed out. When a manipulator is constrained to an environment(force-controlled), geometric instability due to the relationship between the manipulator configuration and the force-controlled direction is shown to be a significant factor in overall system stability. This exploratory study points out a rather intuitive, geometrically based stability factor in terms of an effective system stiffness and analyzes the phenomenon both analytically and graphically. Also, a stiffness control algorithm using the kinematic redundancy of a kinematically redundant manipulator is proposed to improve the overall stability in force control.